Cationic cellulose derivatives are used for dispersants, modifiers, aggregating agents, etc. and are finding wide application. Since cellulose raw materials are highly crystalline and hardly undergo chemical reactions, an operation of reducing its crystallinity to increase its reactivity is need in the production of cationic cellulose derivatives. The method most generally employed is to convert cellulose to alkali cellulose by mixing cellulose with a large amount of water and a large excess of an alkali metal hydroxide in slurry state, i.e., by an activating treatment called ALCELL process or mercerization, and thereafter, allow the alkali cellulose to react with a cationizing agent, such as a glycidyltrialkylammonium chloride, or a hydroxyalkylating agent, such as an alkylene oxide, thereby converting to its derivative. In other known methods, cellulose is dissolved in a dimethylacetamide solvent containing lithium chloride in the presence of a catalyst, such as amine and tertiary alcoholate, and then converted to its derivative (for example, Patent Document 1), or cellulose is mechanochemically decrystallized in advance by using a ball mill, rod mill, etc. and then converted to its derivative (for example, Patent Document 2).